Purpose: To illustrate the classic
experiment of 1676 in which Ole Roemer first determined the finite speed of
light using timings of eclipses of Jupiter's moon Io.

Jupiter's Moons and the Speed of Light,
uses a version of Project CLEA's Jupiter's Moons software.. Students view
a simulated telescopic view of Jupiter and its satellites. After identifying Io,
they record the precise time when the moon enters Jupiter’s shadow at a date
about two months after conjunction. Using the recorded time of this eclipse and
the known period of Io, students predict the time of an eclipse near opposition
and then record the observed time of that eclipse. The discrepancy between the
predicted and observed times, along with the difference in the distance between
Earth and Jupiter at the two eclipses yields a value of the speed of light
accurate to about 10%. Software provided with the exercise enables students to
calculate predicted times and Earth/Jupiter distances, as well as to analyze the
time discrepancy and to visualize the logic of the analysis. A student manual,
including historical and scientific background of the exercise is provided.

Required Windows Software

The Speed
of Light Exercise uses a version of the Jupiter's Moons software,
Click Filename to Download (using
left mouse button to "Save Link As") This is a zip file that
contains an .exe file that will install itself when you run it on
your computer, producing two icons, one for the Roemer Speed of light
exercise, and one for the Mass of Jupiter exercise (see the CLEA listing
and manuals for the Mass of Jupiter exercise by cllicking here).